Current switch circuit

ABSTRACT

A current switch circuit consisting of a couple of transistors, one transistor acting as a reference element and the other as an input element, a pair of series connections of a resistance element and a diode being connected between the respective collectors of the said transistors with the polarity of the diodes opposite to each other, so that the emitter current of the transistors are automatically regulated to maintain a predetermined value, whereby the DC levels of the output voltages of the current switch circuit are kept constant against temperature variation of the transistors.

iJriited States Patent [191 Taniguchi et al.

[ CURRENT SWITCH CIRCUIT [75] inventors: Kenji Taniguchi; lchiroImaizumi,

both of Kodaira, Japan [73] Assignee: Hitachi, Ltd., Tokyo, Japan 221Filed: June 4,1970 211 App]. No.: 43,497

[30] Foreign Application Priority Data June 6, 1969 Japan 44/44009 [52]US. Cl 307/2l5, 307/218, 307/297, 307/310 [51] Int. Cl. 03k 19/30,1-103k 19/34 [58] Field of Search 307/202, 213, 214, 307/215, 218,237,297, 310; 330/23, 30 D [56] References Cited UNITED STATES PATENTS3,259,761 7/1966 Narud et a1 307/297 X 3,518,986 7/1970 Woods et a1...330/69 X 2,999,173 9/1961 Ruck 307/237 3,524,141 8/1970 Montgomery eta1. 330/22 3,437,840 4/1969 Murray et a1 307/291 X 1 Sept. 11, 19733,522,446 8/1970 Kodama 307/215 3,440,449 4/1969 Priel et al. 307/2913,509,362 4/1970 Bartholomew 307/296 X 3,523,194 8/1970 Sheng 307/296 X3,538,348 11/1970 Hillis et al. 307/291 X 3,590,274 Marley 307/310 XPrimary ExaminerJohn W. Huckert Assistant Examiner-L. N. AnagnosAtt0meyCraig, Antonelli and Hill [57] ABSTRACT A current switch circuitconsisting of a couple of transistors, one transistor acting as areference element and the other as an input element, a pair of seriesconnections of a resistance element and a diode being connected betweenthe respective collectors of the said transistors with the polarity ofthe diodes opposite to each other, so that the emitter current of thetransistors 'are automatically regulated to maintain a predeterminedvalue, whereby the DC levels of the output voltages of the current,switch circuit-are kept constant against temperature variation of thetransistors.

11 Claims, 9 Drawing Figures PATENTED 7 3.758.791

SHEET 2 BF 3' INVENTORS KENTl TANIGUCHI AND \CHIRO nwuzum BY c141HnTonQlh, Steamer: mu

ATTORNEYS CURRENT SWITCH CIRCUIT BACKGROUND OF THE INVENTION sistor ofwhich an input signal to the circuit is impressed on the base, bothtransistors being connected with the emitters common.

A typical circuit including the current switch circuit as the basiccomponent is a circuit known as a CML (current mode logic) whichincludes a plurality of input transistors, the respective emitters aswell as collectors of the input transistors being mutually connectedrespectively, so that an OR output can be taken out from the collectorof the reference transistor and a NOR output from the common collectorlead of the input transistors.

The CML in which the transistors are operated in the respectivenon-saturation regions so as not to be affected by the stored charge, isan effective logic circuit compatible to the so-called CTL(complementary transistor logic) especially in the operation speed, Inthe practical use of a CML, a further stage of an emitter follower isconnected to each output circuit of the reference and the inputtransistors soas to reduce the output impedance and to equalize thelevels of the input and output signals. Conventional current switchcircuits including the above-mentioned CML, however, have a commondrawback that the DC voltage levels of the output signals changewithvariation in the temperature at the junctions of the transistors becauseof the temperature dependence of the forward base-emitter voltage. Forexample, with a typical practical CML which is designed so as to have alogic swing voltage of approximately 0.8 V with the l level of theoutput signal set at -0.8 V or so, the 0 level at l.6 V or so and thereference level at l .2 V or so, the 1 level signal is subject to thevariations with a temperature coefficient of about 1.3 to 2.0 mV/C,especially under the influence of transistors of the emitter followers.As for the 0 level signal, the temperature coefficient of the drift isfound to be about 0.5 to 0.8 mV/C which is much lower than that for the1 level signal. This is due to the fact that the 0 level signal isaffected also by the temperature characteristics of the inputtransistors and reference transistors which more or less compensate forthe influence of the drift of the emitter follower transistors.

Anyway, such temperature dependence of the output levels results in apoor immunity of the signal against noise in the conventional CML.Especially, if the conventional CML is fabricated in an integratedcircuit formation, such a circuit would sometimes fail to operate at theintended logic swing voltage, as the component circuit elements are notallowed sufficient heat dissipation. Further, the limit level of theinput signal below which the logic circuit can operate in anonsaturation state, also varies with the temperature coefficient ofabout i .5 to l .8 mV/C in a range of 0.4 to 0.8 V. Therefore, if the 1level signal rises with a temperature rise, it is possible for thesignal level to trespass on the saturation region, exceeding theabovementioned limit level. This necessitates a more limited toleranceof the temperature characteristics of the component circuit elements.

' SUMMARY OF THE INVENTION The object of this invention is to provide acurrent switch circuit of which the DC levels of the output voltages arenot affected by temperature variations of transistors.

In order to achieve the above object, the current switch circuit of thisinvention comprises a reference transistor and an input transistor withthe respective emitters connected together, the common emitter leadbeing connected to an emitter source terminal through a means forregulating the emitter current, output signals being derived from therespective collectors of the said transistors through emitter followercircuits respectively, and further the said current switch circuit isprovided with a pair of additional routes connecting both collectors,each consisting of a series connection of a diode and a resistanceelement, the conducting direction of said pair of routes being mutuallyopposite.

Other objects, features and merits of this invention will be clarifiedby the following description with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of anembodiment of this invention.

FIG. 2 is a characteristic diagram of a transistor showing the relationbetween the temperature coeffi' cient of the base-emitter forwardvoltage and the emitter current.

FIG. 3 is a diagram showing manners of connection of semiconductorelements used in this invention.

FIGS. 40 and 4b are circuit diagrams of two other embodiments of thisinvention.

FIG. 5 is a complete circuit diagram of a logic circuit incorporatingthe circuit shown in FIG. 1

FIGS. 6 and 7 are circuit diagrams showing modifications of a portion ofthe circuit shown in FIG. 5.

FIG. 8 is a circuit diagram of an alternative setup of the circuit shownin FIG. 5

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 which showsa current switch circuit of this invention applied to a CML, referencenumeral 10 designates a reference transistor .to whose base terminal 11is applied a reference voltage V and 20 an input transistor to whosebase terminal 21 is applied an input voltage. It is assumed that theinput transistor 20 represents a plurality of similar transistorsconnected in parallel to receive a corresponding number of input signalsof the CML. The emitters l2 and 22 of the reference and inputtransistors 10 and 20 are connected together and led to the emittersource terminal 31 through an emitter current regulating means 30. Therespective collectors 13 and 23 of the reference and input transistors10 and 20 are connected to the collector source terminal 32 respectivelythrough load resistors 14 and 24 whose resistance values are R and Rrespectively. Another transistor 15 constitutes an emitter follower withthe base thereof connected with the collector 13 of the referencetransistor 10. The

emitter of the transistor is the OR output terminal 16 of the CML. Onthe other hand, still another transistor 25 constitutes another emitterfollower with the base thereof connected with the collector 23 of theinput transistor 20. The emitter of the transistor 25 is the NOR outputterminal 26 of the CML. Reference numerals l7 and 27 respectivelydesignate load resistors connected between the emitter source terminal31 and the emitters of the transistors 15 and 25. Further, a pair'ofseries connections respectively consisting of a resistance element 36and a diode 33, and a resistance element 35 and a diode 34, areconnected between the collector l3 and 23 of the transistors 10 and in amanners that the conducting direction of the series connections areopposite to each other. The temperature compensation for the outputsignal of the CML is acheved by providing these series connections, aswill be described hereinafter.

The circuit shown in FIG. 1 is operated, for example, with the collectorsource terminal 32 grounded, therefore the potential V at the collectorsource terminal 32 being zero Volt and the potential V at the emittersource terminal 3l-being negative. It is assumed in the followingdescription that the input signal V, is at the 1 level and the inputtransistor 20 is conducting while the reference transistor 10 is in theshutoff state. For convenience of the explanation, it is further assumedthat the emitter-grounded current amplification factor B and thebase-emitter forward voltage drop V are identical for all of the abovetransistors, and that the forward voltage drop V of the diodes 33 and 34are identical. Moreover, the base current of the transistors 15 and areneglected in the following explanation.

With the input signal V at the 1 level, the diode 33 is not conductingwhile the other diode 34 conducts, and an output signal V, of the 1level appears at the output terminal 16-while an output signal V., ofthe 0 level appears at the output terminal 26. These output voltages V,and V can be expressed in the following formulas. 1

VI ow/ s RCP RCN) 0 on s) 51;

0 on/ s RCP on) I VD s cP)' a l B)/ ns/ Therefore, the conditions foreliminating the temperature dependence of the output voltage V, and Vcan be determined by regarding that the left side of each of the aboveequations'3 and 4 is zero. That is:

s/ ep D/ BE/ mma mm (WU/ 1 s Rep RCN/RCP BE/ gether the base and thecollector of a transistor as shown in FIG. 3, as is often found insemiconductor integrated circuits, the value of dV /dT can be made equalto that of dV /dT. Therefore, assuming dV /dT dV /dT 2 (mV/C) 7) s RCPow Similarly, from the above-mentioned assumption 7 and the equation 6,

C-- a la/d1 2 (am m Therefore, it will be seen that the requiredstabilizing conditions can be achieved by setting the temperaturecharacteristics of the emitter current 1,; so that the condition of theabove equation 9 is satisfied. Various methods for setting the emittercurrent in such a manner as mentioned above will be shown in thefollowing description of the embodiments of this invention.

Meanwhile, in the conventional CML, the output signal levels V, and Vwould drift at different temperature coefficients. Therefore, thereference voltage source in the conventional CML should be so designedthat the reference voltage V renders an intermediate temperaturecoefficient between those for the levels V, and V 1.1 mV/" C forexample. According to the present invention, however, the referencevoltage source should be designed so as to produce a constant voltage,as both output signals V, and V have zero temperature coefficient.

FIGS. 4a and 4b show further embodiments of this invention. In FIG. 4aare shown the diodes 33 and 34 connected in parallel in oppositedirections, the parallel connection of these diodes being connectedbetween the respective collectors of the transistors 10 and 20 throughresistors 35 and 36. Further, in FIG. 4b, a resistor 37 is seen to besubstituted for the two resistors 35 and 36 in FIG. 4a, the resistancevalue of the resistor 37 being equal to the sum of the resistance valuesR and R of the resistors 35 and 36. Other components shown in FIGS. 4aand 4b correspond to those indicated by similar reference numerals inFIG. 1. Also, the principle and operation of the temperaturecompensation with the circuits shown in FIGS. 4a and 4b are the same asthose described above in connection with FIG. 1. Therefore, repeatedexplanation is omitted.

Referring to FIG. 5 which more tangibly shows a CML incorporating thecircuit of this invention, elements or components corresponding to thoseshown in FIG. 1 are designated with similar reference numerals. Thereference base voltage V of the reference transister 10 is supplied froma biasing circuit 40 indicated by a dot-and-dash line box in FIG. 5.This biasing circuit consists of a transistor 41, biasing resistanceelements. 42, 43, 44 and 45, and an appropriate number of diodes 46 fortemperature compensation, and is composed in a manner that the variationin the baseemitter voltage drop of the transistor 41 due to the changeof the temperature is compensated by a voltage drop in the above diodes46 to thereby make the temperature coefficient for the reference voltageV zero. Further, the emitter current regulating means 30 which isconnected between the emitter source terminal and the common emitterlead of the reference and input transistors, consists of a transistor 50and a resistance element 54 connected to the emitter 51 of thistransistor for providing a feedback voltage. The base 53 of thetransistor 50 is connected to the biasing circuit 40to be driven with avoltage divided by the resistors 44 and 45 in the biasing circuit 40. 1

Defining that R 'is the resistance value of the resistor 54, V, thepotential of the base 53 of the transistor 50, and V the base-emitterforward voltage drop, and further, assuming that the currentamplification factor B of the transistor 50 is sufiiciently high, thecondition CML, that is, may be incorporated in a semiconductorintegrated circuit. Or, it may be a separate unit from the main part ofa CML. Further, various other circuit configurations are possible forthe biasingcircuit 40 so as to be adapted for various emitter sourcevoltages V reference voltages V and other requirements.

FIG. 6 shows one example of such other configurations of the biasingcircuit 40. As is seen in the Figure, a resistance element 47 isconnected across one (46a) of the plurality of temperature compensatingdiodes'46 in order to facilitate accurate matching of thetemperaturecoeffrcients of the reference voltage V,,,, and the drivingvoltage V FIG. 7 is another example of the biasing circuit 40. In thisexample, it will be noted that the driving voltage V, for the transistor50 is not kept constant, as a diode 48 is inserted between the referencevoltage terminal 11 for the reference transistor 10 and the drivingvoltage terminal 53 for the transistor 50. Thus, the circuit is designedso that the reference voltage V is immune to temperature variation,while the driving voltage V,i ncreases with the temperature risebecauseof the for producing an emitter current I, which will satisfy thecondition of the afore-mentioned equation 9 can be determined asfollows. V Y

Firstly, as is seen from FIG. 5, I, (V, V VEE/REE) x a where, a (BIB 1)As the biasing circuit is designed so as to have no temperaturedependence as mentioned previously, the voltage -V, does not change withvariations of temperature. Therefore, substituting the above formula 1into the left side of the equation 9,

Therefore, R /R 2/0: 2 2 (12) This is the condition for setting thevalue of the emitter current I, at the desired level. Thus, in thecircuit shown in FIG. 5, the condition of the equation 9 is satisfled bysetting the resistance of the element 54 ataptemperature dependence ofthe forward voltage drop V of the diode 48. As is clear from FIG. 7,

VJ ss 0 z an as- RCN/REE z 1 Thus, even when the driving voltage V, isvariable against temperature, the temperature dependence of the outputsignallevel can be eliminated by setting the resistance value of theemitter resistor 54 to be nearly equal to the resistance value of theload resistor 24.

"FIG. 8 shows another example of the CML in which the circuit ofthis-invention is incorporated. In this circuit, the base-emittervoltage of the transistor 50 which regulates the emitter currents of thereference and input transistors 10 and 20, is set by a diode 55.connected between the base 53 and the emitter 51 of the transistor 50,and the base 53 is also connected to the reference voltage terminal 11through a resistance element 44. Assuming that the circuit isconstructed in the form of a semiconductor integrated circuit and thatthe ratio of the area of the emitter junction in the transistor 50 tothat of the junction in the diode 55 is 'y, the emitter current I, isabout 7 times as large as the current flowing through the diode 55. Thegreater part of the current I which flows through the resistance element44 (whose resistance value is assumed to be R flows through the diode55. Therefore, assuming that the forward voltage drop in the diode 55 isV,,, the emitter current I; of the transistor 50 is determined by thefollowing formula.

- 14) The value of V is nearly equal to the base-emitter voltage drop Vof the transistor. Substituting the formula 14 into the left side of thepreviously shown equation 9 (note that a is nearly equal to I),

The above relation is the condition for setting the emitter current 1,,in this circuit.

It will be clear from the above description that in the current switchcircuit of this invention, the DC voltage levels of the output signalscan be stabilized against temperature variations by setting the emittercurrents of the reference and input transistors at a proper amount.

Though, in this specification the manner of setting the emitter currentof the current switch circuit has been described under the assumptionexpressed by the previously shown equation 7, it will be clear that theprinciple of this invention can be put into practice regardless of theabove assumption. Further, it will be understood that the emittercurrent regulating means used in the circuit of this invention is notlimited to such types as shown in the above embodiments, and that othertypes of circuits which can render the emitter current to have anappropriate temperature characteristics can also be used as theregulating means.

I claim:

1. A current switch circuit comprising;

at least one input transistor to whose base an input signal is applied;

a reference transistor to whose base a reference voltage is applied andwhose emitter is connected with the emitter of said input transistor;

load resistors connected respectively with the collectors of said inputand reference transistors;

emitter followers connected respectively with the collectors of saidinput and reference transistors;

two diode circuits connected in parallel between the collectors of saidinput and reference transistors, each of said diode circuits comprisinga diode and a resistor connected in series with said diode forcontrolling current flowing through said diode, and wherein saiddiodes-disposed in said two diode circuits are oriented in mutuallyopposite directions with respect to each other; i

emitter current regulating means connected in common with the emittersofsaid input and reference transistors for regulating the emitter currentflowing through the commonly connected emitters, so that said emittercurrent has a certain temperature coefficient; and

a power supply source having a first power terminal connected with saidload resistors respectively and a second power terminal connected withsaid emitter current regulating means for providing a power potentialbetween said first and second power terminals,

whereby the DC. voltage levels of the output signals derived out fromsaid emitter followers changeable due to temperature change arestabilized.

2. A current switch circuit as defined in claim 1, wherein said diodesin said diode circuits respectively provide a forward voltage drop whosetemperature coefficient is substantially the same as that of thebaseemitter forward voltage drop in said input and the referencetransistors, and said resistors in said diode circuits respectively havethe same resistance as that of said load resistors.

3. A current switch circuit as defined in claim 2, which furthercomprises a temperature-compensated biasing circuit which produces areference voltage having a substantially zero temperature coefficientcoupled to the base of said reference transistor.

4. A current switch circuit as defined in claim 1, wherein said emittercurrent regulating means comprises a first transistor whose collector isconnected in common with the emitters of said input and referencetransistors and to whose base a regulation voltage is applied, and meansfor connecting the emitter of said first transistor with said secondpower terminal.

'5. A current switch. circuit as defined in claim 4, which furthercomprises a temperature-compensated biasing circuit including: I

a second transistor having its emitter-collector circuit coupled betweensaid first power supply terminal and the base of said referencetransistor;

a first biasing resistor coupled between the base of said secondtransistor and said first power supply terminal;

at least one diode and a second biasing resistor con- I nected in seriesbetween said second power supply terminal and the base of said secondtransistor; and

a voltage dividing circuit which comprisesfirst and second impedancemeans connected in series between the emitter of said second transistorand the second power supply terminal, the common connection of saidfirst and second impedance means being connected with the base of saidfirst transistor.

6. A current switch circuit as defined in claim 5, wherein said firstand second impedance means are composed of third and fourthbiasing-resistors respectively, and said means for connectingthe'emitter of said first transistor with said second power supplyterminal is composed of a feedback resistor whose resistance value isequal to one-half of that of said load resistors.

7. A .current switch circuit as defined in claim 5, wherein said firstimpedance means is composed of a diode, said second impedance element iscomposed of a biasing resistor, and said means for connecting theemitter of said first transistor with said second power supply terminalis composed of .a feedback resistor whose resistance value is equal tothat of said load resistors.

8. A current switch circuit as defined in claim 5, wherein said firstand second impedance means are composed of a biasing resistor and adiode respectively, and said means for connecting the emitter of saidfirst transistor with said first power supply terminal is composed of aconductor for directly connecting therebetween, the resistance value R,,of said first impedance means being so selected as to satisfy theequation 1% R R /y, wherein R is the resistance value of said loadresistors and y is the ratio of the area of the emitter junction in thefirst transistor to that of the junction in said diode which comprisessaid second impedance means.

9. A current switch comprising:

at least one input transistor having an emitter, a base and a collector;

means for connecting a biasing power supply across the collector andemitter of said input transistor;

first and second output circuits, each of which is connected to receivea signal from the collector of said at least one input transistor and toprovide output signals therefrom, the output of one of said outputcircuits being at a first level while the output of the other of saidoutput circuits is at a level, different from said first output level;and

means, coupled between said pair of output circuits for maintaining thetwo different output levels from each of said first and second outputcircuits substantially constant over a prescribed temperature range,including a pair of pn junction-resistor cir-.

cuits, each of which comprises a pn junction and a resistor connected inseries, said pn junctionresistor circuits being connected in parallel,and wherein the pn junctions are oppositely poled with respect to eachother within said parallelconnected circuits.

10. A current-switch according to claim 9, further inence transistor anda power supply terminal.

* i ll 0

1. A current switch circuit comprising: at least one input transistor towhose base an input signal is applied; a reference transistor to whosebase a reference voltage is applied and whose emitter is connected withthe emitter of said input transistor; load resistors connectedrespectively with the colleCtors of said input and referencetransistors; emitter followers connected respectively with thecollectors of said input and reference transistors; two diode circuitsconnected in parallel between the collectors of said input and referencetransistors, each of said diode circuits comprising a diode and aresistor connected in series with said diode for controlling currentflowing through said diode, and wherein said diodes disposed in said twodiode circuits are oriented in mutually opposite directions with respectto each other; emitter current regulating means connected in common withthe emitters of said input and reference transistors for regulating theemitter current flowing through the commonly connected emitters, so thatsaid emitter current has a certain temperature coefficient; and a powersupply source having a first power terminal connected with said loadresistors respectively and a second power terminal connected with saidemitter current regulating means for providing a power potential betweensaid first and second power terminals, whereby the D.C. voltage levelsof the output signals derived out from said emitter followers changeabledue to temperature change are stabilized.
 2. A current switch circuit asdefined in claim 1, wherein said diodes in said diode circuitsrespectively provide a forward voltage drop whose temperaturecoefficient is substantially the same as that of the base-emitterforward voltage drop in said input and the reference transistors, andsaid resistors in said diode circuits respectively have the sameresistance as that of said load resistors.
 3. A current switch circuitas defined in claim 2, which further comprises a temperature-compensatedbiasing circuit which produces a reference voltage having asubstantially zero temperature coefficient coupled to the base of saidreference transistor.
 4. A current switch circuit as defined in claim 1,wherein said emitter current regulating means comprises a firsttransistor whose collector is connected in common with the emitters ofsaid input and reference transistors and to whose base a regulationvoltage is applied, and means for connecting the emitter of said firsttransistor with said second power terminal.
 5. A current switch circuitas defined in claim 4, which further comprises a temperature-compensatedbiasing circuit including: a second transistor having itsemitter-collector circuit coupled between said first power supplyterminal and the base of said reference transistor; a first biasingresistor coupled between the base of said second transistor and saidfirst power supply terminal; at least one diode and a second biasingresistor connected in series between said second power supply terminaland the base of said second transistor; and a voltage dividing circuitwhich comprises first and second impedance means connected in seriesbetween the emitter of said second transistor and the second powersupply terminal, the common connection of said first and secondimpedance means being connected with the base of said first transistor.6. A current switch circuit as defined in claim 5, wherein said firstand second impedance means are composed of third and fourth biasingresistors respectively, and said means for connecting the emitter ofsaid first transistor with said second power supply terminal is composedof a feedback resistor whose resistance value is equal to one-half ofthat of said load resistors.
 7. A current switch circuit as defined inclaim 5, wherein said first impedance means is composed of a diode, saidsecond impedance element is composed of a biasing resistor, and saidmeans for connecting the emitter of said first transistor with saidsecond power supply terminal is composed of a feedback resistor whoseresistance value is equal to that of said load resistors.
 8. A currentswitch circuit as defined in claim 5, wherein said first and secondimpedance means are composed of a biasing resistor and a dioderespecTively, and said means for connecting the emitter of said firsttransistor with said first power supply terminal is composed of aconductor for directly connecting therebetween, the resistance value RDof said first impedance means being so selected as to satisfy theequation 1/2 RCN RD/ gamma , wherein RCN is the resistance value of saidload resistors and gamma is the ratio of the area of the emitterjunction in the first transistor to that of the junction in said diodewhich comprises said second impedance means.
 9. A current switchcomprising: at least one input transistor having an emitter, a base anda collector; means for connecting a biasing power supply across thecollector and emitter of said input transistor; first and second outputcircuits, each of which is connected to receive a signal from thecollector of said at least one input transistor and to provide outputsignals therefrom, the output of one of said output circuits being at afirst level while the output of the other of said output circuits is ata level, different from said first output level; and means, coupledbetween said pair of output circuits for maintaining the two differentoutput levels from each of said first and second output circuitssubstantially constant over a prescribed temperature range, including apair of pn junction-resistor circuits, each of which comprises a pnjunction and a resistor connected in series, said pn junction-resistorcircuits being connected in parallel, and wherein the pn junctions areoppositely poled with respect to each other within saidparallel-connected circuits.
 10. A current-switch according to claim 9,further including a reference transistor having an emitter, a base, anda collector, the emitter of said reference transistor coupled to theemitter of said at least one input transistor, and the collector of saidreference transistor being connected to one end of said pnjunction-resistor circuit, the other end of said pn junction -resistorcircuit being connected to the collector of said input transistor.
 11. Acurrent switch according to claim 10, further including an emittercurrent regulating means coupled in series between the emitters of saidinput and reference transistor and a power supply terminal.